1. Field of the Invention
The invention relates to a method for analysing tire vibration characteristics and an apparatus for carrying out said method.
2. Description of the Related Art
The tyre is the element through which the road and the vehicle exchange forces. If the structure of the tire is rigid, forces are transmitted directly, without any attenuation. If the structure is elastic the forces transmitted can be attenuated, or on the other hand amplified, due to resonances. The tire is a viscoelastic body and, like all viscoelastic bodies, it vibrates when it is excited (by forces or by deformations). It is therefore possible to say that the vibrations felt by the passengers inside the vehicle are influenced or generated by the tire.
Today, development of a tire for a vehicle model is made in accordance with detailed specifications of car manufacturers. During development specific changes with respect to the construction and the materials used have to be made in order to influence the performance of the vehicle. A specific change of tire parameters helps to save development time and costs. In this connection the analysis of the vibration characteristics, in particular the natural vibrations of tires, is important. Nowadays, this analysis is carried out using testing benches.
In known testing benches comprising a wheel support for supporting a wheel with a tire to be analysed the excitation of vibrations is made via the tire surface. In this case a superposition of free tire vibrations with friction effects cannot be excluded.
In another known testing bench as disclosed in KR-A 1020030043366 the introduction of vibrations is made via an element which is arranged in the surface of the tire with positive locking. A vibrator having a stinger at an upper end thereof is mounted on a lower plate of a supporting frame. A wheel with a tire fitted thereon is suspended from levers which transfer vibrations to an accelerometer arranged at the upper plate of the supporting frame. Since the introduction of vibrations is made via the surface of the tire, modulations can occur due to local bending stress.
EP-B1 0 775 902 and 0 775 904 disclose a torsional vibrator capable of torsionally exciting a tire of a wheel and providing measurements of angular vibrations. The wheel with the tire fitted thereon is fastened on a driving shaft of the vibrator. Two acceleration transducers are fastened to the rim in a diametrically opposite position. Two further acceleration transducers are fastened to the tire by means of a clamp fastened to the tire surface. In operation the driving shaft performs angular oscillations and torsionally excites the wheel that thus vibrates in a rotational direction. The accelerometers detect rotational vibrations and provide measurements of angular acceleration of the wheel and of the tire. This known apparatus only permits analysis in one functional direction i.e. rotational vibrations.